Molded articles made from polyolefin-based foamed resin particles are used for a wide variety of applications such as cushioning materials and packaging materials. In recent years, such molded articles have been required to have antistatic properties or electrical conductivity for use as packaging or shipping materials for electronic components such as semiconductors.
A method for imparting antistatic properties or electrical conductivity to molded articles includes adding a conductive material into foamed resin particles as raw materials for the production of the molded articles. Known examples of such conductive polyolefin-based foamed resin particles include conductive polyolefin-based foamed resin particles containing conductive carbon black dispersed therein.
A large amount of conductive carbon black is necessary for high electrical conductivity. However, a polypropylene-based resin has high crystallinity. If the large amount of conductive carbon black is dispersed in such a polypropylene-based resin, bubbles (hereinafter called “cells”) in the polypropylene-based resin are caused to be extremely fine due to the high crystallinity of the polypropylene-based resin. This causes the resulting foamed particles to easily decrease in internal pressure. This reduces the foaming power and makes the molding very difficult (due to shrinkage of molded articles, formation of surface wrinkles, and other defects), which means that it is difficult to obtain polypropylene-based foamed resin particles with excellent physical properties.
Previously, a method of foaming foamable resin particles produced by impregnating, with a foaming agent, polypropylene-based resin particles containing a carboxyl group-containing polypropylene-based resin as a base material, conductive carbon black, and a bubble nucleus; and a method of foaming foamable resin particles produced by impregnating, with a foaming agent, polypropylene-based resin particles containing conductive carbon and a water-soluble inorganic substance were proposed (Patent Literature 1).
Unfortunately, in molded articles containing a large amount of conductive carbon black, the conductive carbon black can easily drop off from the molded articles and easily cause contamination on products packaged therein. The foamed resin particles obtained by foaming the foamable resin particles containing conductive carbon black, and a bubble nucleus or a water-soluble inorganic substance still have fine cells and poor moldability. Therefore, foamed molding articles obtained from the foamed resin particles do not have a satisfactory level of surface appearance, expansion, color uniformity, or other properties.
It is further disclosed that even when containing a large amount of conductive carbon black, polypropylene-based resin particles of a polypropylene-based resin composition containing a polypropylene-based resin and a certain amount of a bubble size enlarging agent such as polyethylene glycol or glycerin can be foamed to form a product having large and uniform cells and improved flame retardancy (Patent Literature 2). However, the technique disclosed in Patent Literature 2 still has room for improvement in preventing the dropping off of conductive carbon black.